This invention relates generally to test instruments for local area networks and in particular to a circuit for determining the speed of a local area network.
Increasingly complex LANs now connect more and more types of devices including personal computers, work stations, file servers, and printers. Network hubs are often the central devices in a network through which information flows. Each client device connects to the LAN via adapters called network interface cards (NICs) to form nodes. Connecting the nodes to the hubs are network links which may be unshielded twisted pair (UTP) wire, coaxial cable, or fiber optic cable.
Network protocols for controlling the communication of information between the nodes have been developed, the most common being Ethernet or 10BASE-T which is defined according to the IEEE 802.3 standard. Ethernet has a speed of 10 megabits per second and uses a media access protocol called carrier-sensing multiple access with collision detection (CSMA/CD) to control information traffic flow and resolve collisions between nodes. A node can send information on the network only if no other node is currently sending information. If a node tries to send information at the same time as another node, a collision occurs and each node operates according to a well-defined "back off" procedure to resolve the collision. Each node will wait a random period of time to attempt to send the information again.
Because Ethernet is typically implemented in a baseband, broadcast network, every node receives the information sent by every other node within the collision domain. In order to minimize the burden on the software operating in host personal computers (PC's) connected to the network, a hardware layer with a hardware or media access control (MAC) address passes along to the software layer only the information appropriate for that node. Such information may be in the form of a "broadcast" message intended for all nodes in the network or as a message only for the intended node with the MAC address.
Information sent over an Ethernet network is in the form of discrete packets defined according to the seven layer Open Systems Interconnection (OSI) standard maintained by the American National Standards Institute (ANSI). OSI is a layered structure in which the highest layers take advantage of the capabilities of the lower layers to send information between nodes. Information is passed between nodes in the form of discrete packets containing data or control information supplied by the various OSI layers. The highest layers are the Application layer, the Presentation layer, and the Session layer which may include Telnet, File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS).
The Transport layer typically includes the Transmission Control Protocol (TCP) along with the User Datagram Protocol (UDP), among others, which provide for the delivery of the data to a desired device and the division of the information into discrete packets for sending. Received packets are reassembled in a like manner. The Network layer routes messages back and forth between a source node and a destination node according to Internet Protocol (IP) addresses by adding an IP header to each packet indicating the source and destination IP addresses. The lowest layer is the physical link layer in which the hardware MAC addresses are used.
Fast Ethernet is a new local area network standard commonly known as 100BASE-T which is specified under IEEE 802.3u to provide a ten-fold increase in speed over the original 10BASE-T Ethernet while maintaining the same media access protocol. While the packets are sent faster over a 100BASE-T link, the network layer and transport layers are otherwise unaffected by the speed difference, allowing for LANs containing both 10BASE-T and 100BASE-T links. Because of the considerable investment in 10BASE-T networks and the interoperability between the 10BASE-T and 100BASE-T standards, many LANs will consist of both 10BASE-T and 100BASE-T links.
The 100BASE-T links will typically used in links requiring the most data-carrying capacity to resolve network congestion problems, such as between a shared hub and a file server. The 100BASE-T links require higher bandwidth twisted pair wiring which conforms to the industry-standard known as Category 5 as specified under EIA/TIA 568-A for building wiring standards. The presence of both speeds on the same LAN has given rise to dual speed network hardware that can be readily configured to handle either speed.
Network interface cards, shared hubs, and other LAN hardware are now commercially available that allow for automatically switching between 10BASE-T and 100BASE-T (dual speed 10/100). In both 10BASE-T and 100BASE-T networks, basic connectivity between devices on a link is established and maintained by the presence of the link pulse. A shared hub will provide a link pulse that is received by the NIC upon boot-up. Loss of the link pulse signals the other device that the communications link has been lost. Network traffic is not normally forwarded on a link that has not established communications so the link pulse is often the only signal available without using the chipsets to initiate communications. Link pulses for 10BASE-T and 100BASE-T networks have substantial differences in their frequency characteristics.
As specified under IEEE 802.3u, clause 28, auto-negotiation allows for dual speed adapter cards to be installed in a network without having to change settings manually. Each adapter card typically contain one or more chipsets to handle the task of communication with the LAN. Each time the adapter card is booted up, the auto-negotiation algorithm first exchanges information with the device on the far end of the link to determine common operating modes and then selects the highest priority mode for the chipset to communicate with. The link pulse sent by a dual-speed adapter card is known as an "N-Way Link Pulse" that allows 10BASE-T, 100BASE-T, or dual-speed adapter cards to properly begin communication.
Auto-negotiation between dual-speed devices, in order to operate properly, requires the use of chipsets that are powered up and operating with compatible communication modes on both ends of the link. Auto-negotiation further requires that the installed cable comprising the link be configured according to a set of presumptions about the cable wiring. However, in a test instrument for a LAN, such assumptions may not be valid, requiring more versatility and improved capability for proper configuration. Cables may be misconfigured and have different configurations depending on the devices at each end of the link, including hub-to-NIC connections and hub-to-hub connection. Battery consumption is a significant concern in a handheld portable test instrument, making it desirable to operate the relatively high power-consuming chipsets only when needed. Therefore, it would be desirable to provide a circuit for detecting the speed of a LAN in a test instrument using a circuit by analyzing the frequency content of the link pulse before powering up and connecting the appropriate chipset.